1. Technical Field
The present invention relates to an apparatus for sensing air-fuel ratio which senses air-fuel ratio of air-fuel mixture gases supplied to a combustion apparatus on the basis of oxygen content in the exhaust gas of a variety of combustion apparatuses, e.g., an internal combustion engine.
2. Prior Art
For instance, the Published Unexamined Japanese Patent Application No. 59-178354 discloses a typical air-fuel ratio sensor which senses air-fuel ratio of air-fuel mixture gases supplied on a combustion apparatus such as internal combustion engine by detecting the oxygen content in the exhaust gas. In this case, a porous electrode is formed to both sides of the oxygen ion conductive solid electrolyte in order to complete a sensor element, a pair of such sensor elements are arranged opposite to each other with a gap therebetween. The gap forms a gas-diffusion regulation chamber in which diffusion of exhaust gas is limited.
This air-fuel ratio sensor operates one sensor element as an oxygen concentration cell element, and the other sensor element as an oxygen pump element. The ar-fuel ratio sensor of this type provides, on one hand, a measuring circuit which controls a current flowing into the oxygen pump element so that a voltage generated at the electrodes of the oxygen concentration cell element becomes constant. The air-fuel ratio is determined from such a current value. On the other hand, another type of oxygen sensor provides a measuring circuit which makes a constant current flow into the oxygen pump element in such a direction as pumping the oxygen in the gas-diffusion regulation chamber to the circumference thereof. The air-fuel ratio, is determined from a voltage generated at the electrodes of the oxygen concentration cell element.
The oxygen ion conductive solid electrolyte which forms the sensor element increases mobility of oxygen ion with increase in temperature, and a mobility of the oxygen sufficient for normal operation of the oxygen concentration cell element or oxygen pump element is achieved at the temperature higher than a certain value. On the other hand, a voltage generated at the electrodes on both sides of the oxygen concentration cell element or a current flowing into the oxygen pump element changes in accordance with change in ambient temperature. Therefore, the air-fuel ratio sensor is usually provided with a heater formed by a heat-generating resistor, etc., for the purpose of activation of the sensor element and temperature compensation, and heats the sensor element by applying a voltage to the heater from the measuring circuit.
Therefore, an apparatus for sensing air-fuel ratio which is provided with the air-fuel ratio the sensor is capable of easing the effect of temperature change of sensor element by the heater even in case of ambient temperature changes and thereby stably senses the air-fuel ratio. However, this apparatus provides a problem, when heat-generated by the heater changes due to fluctuation of voltage applied to the heater, that the air-fuel ratio cannot be sensed stably because of resultant change in the temperature of the sensor element. Accordingly, the prior art is required to accurately control a voltage applied to the heater and thereby results in a problem that a special electrical circuit for controlling the heater voltage must be provided additionally.